Metal-insulator transition of the reduced surface of yttria-stabilized zirconia near Pt electrodes.

The electrochemical reactions of solid oxide fuel cells occur in the region where gas-phase species, electrode, and electrolyte coincide. When the electrode is an ionic insulator and the electrolyte is an electronic insulator, this triple phase boundary is assumed to have atomic dimensions. Here we use photoemission electron microscopy to show that the reduced surface of the electrolyte yttriastabilized zirconia (YSZ) undergoes a metal-insulator transition near Pt negative electrodes. YSZs electron conducting region functions as an extended triple phase boundary that can be many microns in size, depending on oxygen pressure, temperature, applied voltage, and time.